Steam Cleaning

David Odgers

A small industrial unit being used to clean carved stonework.
The run-off is being collected using a sponge.
In enclosed
spaces the increase in humidity may become an issue.
(Photo: Humphries & Jones)

The cleaning of historic buildings
has been and continues to be a subject
that attracts considerable debate; on
the one extreme there are the exponents who
believe that cleaning is a necessary part of any
conservation process and, at the other end
are those that hold that cleaning only causes
damage and should be avoided. Of course
the answer lies not in any pre-determined
prejudices but rather in a proper assessment of
the building and a well thought out conclusion
as to what is in its best interest.

PRINCIPLES

Although it is sometimes thought that the
cleaning of external elevations of buildings
will slowly become unnecessary due to
reduction in sulphur dioxide levels, there is
a corresponding increase in nitrous oxides
which can act as a catalyst for other reactions
including the deposition of sulphates. It
is inevitable therefore that resoiling will
continue and correspondingly unlikely that
the debate on cleaning buildings will cease.

What is clear is that methodology for
cleaning masonry and brickwork can have a
lasting effect on the ongoing behaviour – and
indeed survival – of the substrate. In some
cases wholly inappropriate methods (such as
sand blasting) have been used with obvious
damage to the stone; such interventions are at
the heart of the continued reluctance to clean
sandstone buildings in Scotland.

Even well-intentioned cleaning
programmes can have a long-term effect that
might not be foreseen. Schaffer (writing in
1932 – see Recommended Reading, below)
reports on the benefit of regular water
washing with water and cites the example
of Goldsmiths Hall in Gresham Street,
then being washed twice a year: ‘the clean
appearance and good condition of the
Portland stone’ he states, ‘are unquestionable’.
And yet recent assessment of some of the
public monuments in London (including the
Cenotaph, illustrated on the next page) has
shown that regular maintenance cleaning has
led to the excavation of the surface pores and
colonisation by microbiological growths that
are very difficult if not impossible to remove.

A decision to clean on aesthetic grounds
is rarely sufficient. In all cases, before any
decision on cleaning is made, a thorough
assessment should take place to:

identify the substrate, its condition,
and its vulnerability to cleaning

understand the nature of the soiling
remembering that this will differ
according to location, orientation
and local environment

decide whether the soiling is
superficial or ingrained

establish whether the soiling is
causing damage to the substrate

ascertain whether cleaning is necessary
to allow other treatments.

As a nationally important monument, the Cenotaph
is regularly maintained. The grey appearance at
the top is not surface dirt but dead microbiological
material that now lies deep within surface pores
that have been excavated by repeated cleaning with
pressurised water. (Photo: Odgers Conservation
Consultants (OCC))

The results of steam cleaning on a typically soiled
section of Bath stone; the steam has removed surface
dirt and the flexible microbiological materials but the
more brittle sulphation layers remain.
(Photo: OCC)

Once these issues are understood, further
consideration must be given to what the
advantages and disadvantages of the removal
of soiling might be, including the likelihood
and rates of re-soiling. Even after all this,
it is also essential that small scale, well
documented cleaning trials take place to
identify the appropriate methods and to allow
all parties to understand what result can be
achieved without damage to the substrate.
Since the rather uncontrolled cleaning
blitz of the 1960s and 1970s where it was
customary to see thousands of gallons of
water being poured indiscriminately down
the elevations of buildings, there have been
considerable developments in the techniques
and materials available for cleaning masonry.

THE OPTIONS

The cleaning techniques broadly fall into four
categories; mechanical, water-based, chemical
and laser radiation. In practice a combination
of techniques is often useful, and most
buildings would require more than one method
to deal with the different types of soiling.

Mechanical cleaning includes simple
brushing and vacuuming but principally refers
to specialised forms of abrasive cleaning. The
most commonly used types are micro-air
abrasive, dry air abrasive or wet air abrasive.
All of these rely on the use of compressed air
and aggregate; the latter also includes water.
The parameters of all these constituents
can be varied (for example air pressure,
size and nature of aggregate) so the method
is sufficiently versatile to deal with many
different types of soiling. In practice, it is
used mostly for brittle soiling and coatings
on limestone, sandstone, brick, concrete and
granite but it is generally not advisable for
polished surfaces.

Water-based cleaning includes steam
cleaning, sponging, intermittent nebula sprays
(which create a fine mist to slowly soften the
dirt layer), water/clay poultices, rinsing and
pressure washing. Water is an effective solvent
and can be used hot or cold, and as a liquid or
vapour. It is suitable, in limited quantities, for
most substrates, and it is particularly useful
for removing sulphate crusts from limestone,
for some coatings, for superficial deposits and
surface biological growths. The simplicity of
water-based cleaning can be appealing but the
use of too much water can lead to substantial
risks of residual staining, mobilisation and
re-crystallisation of salts, and corrosion of
hidden metal cramps.

Chemical cleaning agents include acids,
alkalis, solvents, chelating agents, biocides
and detergents. They can be delivered to the
surface either as liquids, gels or poultices,
the advantage of the latter being that there is
a longer contact time. All chemicals rely on
breaking down the bonds within the soiling
or between the substrate and the soiling.
There has been considerable investment
in developing targeted combinations of
chemicals that deal with specific types of
soiling on particular substrates. All of these
must be used with care, and most of them
require neutralisation or rinsing with water
afterwards: this factor must be taken into
account when specifying their use. Their
effect on adjacent materials (for example glass,
metals and timber) must also be considered.

Laser cleaning is beginning to have
a wider impact in the UK although the
machinery remains expensive. The method
works on the principle that the dirt absorbs
enough energy from the beam to lose cohesion
and vaporise; so laser cleaning is most
effective when there is a contrast between the
dark soiling and pale substrate.

No matter how benign the technique and
methodology chosen, it is always necessary to
carry out a thorough initial assessment and
trials. As with all other areas of conservation,
it is of course not the machinery or the
materials that are the ultimate reason for
successful cleaning but rather the skills of the
person using them.

STEAM CLEANING METHODS

Of the methods mentioned above perhaps
the most accessible and most widely used is
steam cleaning. Steam cleaners have been in
use since the early part of the 20th century.
Shaffer refers to the use of steam cleaners to
clean a ‘blackened frontage’ and goes on to
say ' …the steam process is unlikely to cause
any more damage than washing with water
or scrubbing with stiff brushes'. In truth, it
is now recognised that steam cleaners cause
much less damage than those methods.

There are, however, many different types
of steam cleaner available and they should be
distinguished from hot water washers. Hot
water has a lower surface tension than cold
and thus is more likely to clean more deeply
and quickly.

A trial area of paint removal from faience; steam
cleaning was used to soften and remove most of the
paint from the substrate. This was followed up with
a paint softener (on the right hand side of the panel)
and a final rinse with steam. (Photo: Restorative
Techniques)

This principle is at the heart of hot
water washers which have diesel fuelled
boilers and a pump that delivers water at
temperatures up to 90°C through a restrictive
nozzle which increases the velocity of the
water. This results in pressures of between
60 and 150 bar and water-use of between
5 and 20 litres per minute. These can be
used in conjunction with detergents or other
chemicals but in reality, this is rare for historic
buildings. More often than not, hot water
washers are the method of choice for rinsing
after chemical cleaning and for removing
algae and other materials from paving.

Some of the machines used in hot water
washing can result in quite aggressive cleaning
because of the high water pressure and volume
they can deliver. Apart from these, other
parameters which can provide some control
include the design of the nozzle, the angle of
spray to the surface being treated, distance
of the spray to the surface and the duration
of contact. All of these can be manipulated
by the operator so it is possible to carry
out careful cleaning using lower pressures,
keeping the nozzle at a greater distance from
the substrate and ensuring the nozzle spray
angle is above 35°.

Steam cleaners can broadly be divided
into small industrial/domestic units and
the larger machines (such as Doff and
ThermaTech) that are commonly encountered
in building conservation.

SMALL UNIT SYSTEMS

The domestic steam cleaner units that are
available at the local DIY store come with a
variety of attachments (including brushes and
nozzles). These however have been devised
mostly for upholstery cleaning and tend
not to be sufficiently robust or to develop a
consistent temperature of steam. They do however have some similarities with the small
industrial steam units (see title illustration)
that are used in conservation; these emit
very small quantities of water (typically 3 to
4 litres per hour) at a pressure of 4 to 6 bar
through handheld nozzles. They are used for
cleaning intricate carved detail, sculpture and
monuments. They are effective on marble but
can dull the surface; they should not be used
on alabaster.

Most of these machines produce wet
steam in which there are also droplets of hot
water. The pressure comes from the steam
generation process itself. As the vapour is
generated, the pressure inside the vessel builds
up; steam at 160°C remains a liquid as long as
the pressure in the container is above about 7
bar. When the pressure is released on opening
the nozzle, the liquid water will vaporize into
steam and cool to the boiling point of water at
atmospheric pressure (100°C). In doing so, it
will expand by about 1.5 times; this expansion
occurs in the nozzle and helps to provide
the pressure of the steam. The temperature
of steam will tend to drop quickly after the
vapour exits the nozzle and some of it will
condense into water droplets. Steam cleaning
using these small machines is effectively a
combination of mostly steam but including
some droplets of hot water; there tends to
be some water run-off that will need to be collected usually by means of a sponge held
beneath the nozzle.

There are now small machines that
generate ‘dry steam’. These heat water up to
higher temperatures (180°C) which under
pressure means that water has effectively
become a gas which is invisible as it exits
the nozzle. Although there will be some
conversion to vapour and a small amount of
condensation on the surface being cleaned, the
heat of the dry steam is sufficient to convert
that liquid to vapour; as a result, there is very
little run-off.

LARGE UNIT SYSTEMS

A large steam cleaner being used to clean ashlar with a 45° fan nozzle (Photo: OCC)

The larger steam cleaning machines have been
designed for site use and operate using an
electric pump to pressurise the water and a
diesel-fired heat exchanger to heat the water.
The resulting combination of superheated
water and steam has a temperature typically
between 120° and 150°C and a flow in the range
of 3 to 10 litres per minute, with a nozzle
pressure of 30 to 150 bar. Although this is
similar to hot water pressure washers, the use
of an atomizing nozzle that diffuses the jet of
steam can result in a very low pressure at the
surface being cleaned. In general, the wider
the spray angle, the lower the pressure at the
substrate and a spray angle of 40° is standard
(see illustration above). A narrower angle can
result in greater pressure that can be sufficient
to cause damage to soft or decayed limestone
and sandstone.

Nozzle selection plays an important part
in the way in which the steam cleaner works.
A range of spray shape and angle is available;
commonly used might be solid cone (suitable
for carved surfaces) and fan shape (suitable
for cleaning larger areas of flat ashlar). But
these nozzles also have different properties
in terms of the diffusion temperature of the
jet of steam; for example a standard nozzle
might lose sharpness at temperatures above
140°. This variable can be used to control the
precision and effectiveness of the jet.

Although some steam cleaners come with
the option of adding chemicals or detergents,
control of the amounts is difficult and in most
cases chemicals require a certain dwell time
to work and this is not provided by including
them in a jet of steam.

RELATIVE MERITS

Steam cleaning is simple and safe as long
as appropriate precautions are taken. The
advantage of steam is its heat; it can be
used generally for flexible materials such as
microbiological growth (algae for example)
and paint coatings. It also has the advantage
over hot water washers that less water is used
and therefore the process is easier to control.
However, it is not generally suitable for brittle
soiling such as calcium sulphate which, in any
case, is less soluble in hot water than in cold.

The use of a vacuum head which collects the steam
and the dirt; this is very useful for large areas of
flat ashlar particularly inside buildings. (Photo:
Restorative Techniques)

There are some drawbacks to the use of
steam cleaners. Anybody who has tried to
take a picture of a steam cleaner in use will
realise that the steam generated can also
make it difficult for the operator to see the
result of their work. In closed spaces, the
increase in humidity may be an issue so good
ventilation is normally essential. However,
there have recently been developments in the
use of vacuum heads that both deliver the
steam to the surface and collect the residue
(see illustration below left); this enables use
in sensitive environments and allows the
operator to see what is happening.

The larger machines will also
generate fumes from the diesel so this too needs to be taken into consideration
when choosing a system.

The way in which a steam cleaner is used
by the operator can make all the difference
to its effectiveness. In many cases, the need
for speed can lead to the nozzle being held
too close to the surface; this can result in
damage to the surface and uneven cleaning.
In many cases, the best cleaning is achieved
using a double-pass technique. The first
pass is at lower pressure and will soften the
soiling; after a period (which might be up
to a few hours) to allow this softening to
happen, a second pass will allow the soiling
to be removed more completely and without
the need for the nozzle to be held close to
the surface. In all cases, the operator should
carry out trials to ascertain the optimum
parameters (such as pressure, temperature
and nozzle type) that best suit the condition of
substrate and the type of soiling.

Steam cleaning is an important element
in the range of options that are available for
cleaning masonry and brickwork. Even as
the machinery gets more refined, there still
remains the fundamental need to understand
the likely short and long-term effects of the
cleaning on the substrate. Each case must be
treated on its merits and always there must be
an underlying criteria to ‘do no harm’.

Author

DAVID ODGERS trained at Wells Cathedral
under Professor Robert Baker. He was a
founder of Nimbus Conservation Ltd in 1984
and its managing director from 1991 to
2005, during which time he was responsible
for repair and conservation works to many
important historic buildings, monuments
and sites. Since 2005 he has been an
independent consultant on all aspects of
conservation works to stone and plaster.
He was the editor for the Stone volume
of English Heritage’s Practical Building
Conservation series. He is senior tutor for the
building conservation diploma at West Dean and is an accredited conservator.